1. Field of the Invention
The present invention pertains, in general, to an optical printed circuit board (PCB) for long-distance signal transmission and, in particular, to an optical PCB for long-distance signal transmission, which is combined with fiber blocks and pipe blocks embedded in opto-via holes to form a multi-layered optical PCB, and transmits an optical signal using a connection member such as a fiber ribbon when an optical signal transmission distance between layers constituting the multi-layered optical PCB is longer than a length of an optical waveguide element used in conjunction with a silicon substrate.
2. Description of the Prior Art
As well known to those skilled in the art, PCBs are circuit boards in which various electronic elements are densely mounted on a surface of a flat plate consisting of a phenol or epoxy resin, and electrically connected to each other. A method of manufacturing such a PCB includes attaching a copper foil to one side of the phenol or epoxy resin insulating substrate, etching the resulting substrate to form a desired circuit pattern (a portion other than the circuit pattern is eroded and removed), and piercing via-holes for receiving the electronic elements on the etched substrate.
Further, the PCBs are classified into three types according to the number of patterned layers constituting the PCB: single-sided PCBs, double-sided PCBs, and multi-layered PCBs. The higher the number of the patterned layers is, the higher the number of electronic elements mounted on the patterned layer will be. Of the three kinds of PCBs, accordingly, the multi-layered PCBs are applied to sophisticated products, and each comprise three or more layers with a conductive pattern in such a way that insulating layers alternate with the patterned layers.
Meanwhile, a conventional PCB comprises a copper plate on which a circuit pattern is formed, and an inner and an outer layer. However, recently, an electro-optical circuit board (EOCB) is frequently used instead of the conventional PCB. At this time, the EOCB is formed by inserting an optical waveguide into the PCB, and the optical waveguide for transmitting and receiving optical signals comprises polymer and glass fiber. In other words, the EOCB is a PCB into which the optical waveguide and a glass plate are inserted after the circuit pattern is already formed on the PCB, and it transmits and receives both electrical signals and optical signals. Additionally, the EOCB functions to convert optical signals into electrical signals so as to store data or process signals in an element mounted on the PCB, and very high-speed data telecommunication is realized when the electrical signal is interfaced with the optical signal in the PCB.
Various coupling methods have been suggested to smoothly transmit and receive optical signals between layers of the multi-layered PCB, and examples of the coupling methods include a direct writing method, a beam reflection method, a reflecting mirror method, and a direct coupling method.
A detailed description of the optical interfacing in the conventional PCB will be given, below.
FIG. 1 schematically illustrates a beam coupling in the case of using a beam reflecting micro mirror.
With reference to FIG. 1, an electrical signal transmitted from a processor board 2 is converted into an optical signal by a laser diode 1 in a transmitting module 3, transmitted via left lenses 8a, 8b, and then reflected by a left micro mirror 4a inserted into the PCB. The reflected signal is transmitted through the optical waveguide, reflected by a right reflecting mirror 4b, and then transmitted through right lenses 8c, 8d to a photo diode 6 in a receiving module 7. At this time, the optical waveguide is embedded within a low loss multimode polymer waveguide core 5a, 5b, and a waveguide clad 9 is formed on an upper and a lower part of the core. Therefore, the electrical signal transmitted from the left processor board 2 is converted into the optical signal and transmitted to a photo diode 6, and the optical signal is then converted into the electrical signal by the photo diode 6 and transmitted to a right processor board.
Hereinafter, there will be described the conventional multi-layered PCB in which the optical signals are transmitted between layers constituting the PCB.
FIGS. 2a and 2b are a front and a side sectional view of the conventional multi-layered PCB in which the optical signals are transmitted between layers of the PCB, respectively.
Referring to FIGS. 2a and 2b, transmission of the signals between layers of the PCB is realized in such a way that a beam emitted from a vertical-cavity surface-emitting laser 13 (hereinafter, referred to as “VCSEL”) acting as a light source element is focused by a micro-lens 17, and then transmitted through opto-via holes 16 of the PCB into the optical waveguide 14, 15. At this time, a silicon optical bench 12 (SiOB) is formed on the PCB 11. The SiOB 12 is a common designation of silicon wafers, and is applied to an optical communication module used to package optical elements. Alternatively, a polymer substrate may be used instead of the SiOB 12. The optical waveguide consists of a clad 14 and cores 15, the optical signals 19 emitted from the VCSEL 13 through the micro-lens 17 are transmitted to the optical waveguide positioned on another layer of the PCB. At this time, the opto-via hole 16 is insulated with an insulating material 18. Furthermore, the micro-lens 17′ may be embedded in the opto-via hole 16 so as to improve the transmission of the optical signals.
The VCSEL 13 is used as a light source of an optical module for transmission and amplification of light source data, in which a circular laser beam is perpendicularly emitted to a surface of the substrate. Conventionally, an LED and an edge-emitting laser diode have been widely used as the VCSEL and, since 1990, a surface-emitting laser (SEL) has been used in optical fiber communication, interfacing, and parallel processing of mass-information instead.
However, the conventional VCSELs are disadvantageous in that the micro-lens 17 should be used if the transmission of the optical signal 19 is conducted through the opto-via holes 16, and their maximum allowable wavelength is 200 μm. Furthermore, it is difficult to embed a curved reflecting mirror or prism used in coupling of waveguide elements into the multi-layered PCB, and there is a problem of misalignment between the curved reflecting mirror or prism and the PCB when a plurality of mechanical drills are used in parallel to pierce the multi-layered PCB so as to connect multi-channel signals to each other. In other words, it is difficult to limit a light emitting distance from the VCSEL or photo diode within a range of 60 to 200 μm in a free space and to locate the micro-lens, the curved reflecting mirror, or the prism in the proper place so as to connect the VCSEL or photo diode to the optical waveguide or optical fiber in the multi-layered PCB.
Additionally, it is impossible to transmit the optical signal between locations more than 30 cm apart because a length of a silicon substrate, that is, a wafer, is 30 cm or less when a plastic polymer used as a material of the optical waveguide is spin-coated on the silicon substrate.